Add a tight OpenCode adapter with run-scoped state isolation #662

Part of #653. Smithers already ships the OpenCode harness in its built-in registry, and the OpenCode + OpenRouter pairing has already driven a full ultrafuzz run at ten concurrent agent seats with opaque catalogue model IDs, using nothing but the stock Smithers agent constructed with a model string, yolo mode, --pure and an output byte cap. The pairing is proven; the isolation is not.

Problem

That run wrote all of OpenCode's state into the operator's real home — config directory, database and write-ahead log, snapshots, tool output, cached model catalogue and downloaded binaries. No ultrafuzz run may write harness state outside a run-scoped root.

Scope

Add packages/runtime/src/templates/smithers/agents/opencode.tsx on the claude.tsx template (~90 lines):

• Construct the Smithers OpenCode agent with the profile model and extraArgs: ["--pure"] (no external plugins).
• Point every state root at a run-scoped directory through env: workflowControlChildEnvironment(...): the XDG config/data/cache/state roots plus OpenCode's own config-dir variable, and the suppressors for autoupdate, share, model-catalogue fetch, default plugins and project config.
• Merge over the env the Smithers agent already sets for yolo mode rather than replacing it — replacing it silently disables permission bypass.
• Read only auth / api_key_env / config_dir from [agents.OpenCodeAgent] via the shared TOML reader off ULTRAFUZZ_CONFIG_PATH.
• Keep the subclass to the buildCommand env scrub.

Register in agentFactories and AGENT_TEMPLATES; add OpenCodeAgent: "opencode" to AGENT_EXECUTABLES in packages/runtime/src/doctor.ts with a version-pin assertion against the qualified version. Ship an opt-in [models.opencode] profile, non-default.

Acceptance criteria

• Adapter is under ~100 lines and overrides only buildCommand.
• Filesystem test: run a smoke prompt with the home directory pointed at a temporary root and assert nothing is created under OpenCode's default config, data or cache locations.
• Unit test: the credential reaches the child env and never the argv.
• Unit test: the agent's own yolo permission env survives the adapter's env merge.
• ultrafuzz doctor requires the pinned OpenCode CLI.

Dependencies

None. The previous dependency on #659 is removed — the pairing needs no further qualification and the isolation work stands alone.